Android 动态添加 Scrollview-Content

Question

我正在努力为滚动视图动态添加一些内容。如果有人能给我一个解释，为什么我的应用程序崩溃，我会很高兴。

public class LessonView extends AppCompatActivity {
    int maxWidth = 450;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // old
        ScrollView SView = new ScrollView(this);

        // new
        // LinearLayout activityLL = (LinearLayout) findViewById(R.id.myLL);

        RelativeLayout.LayoutParams centerTableParameters = new RelativeLayout.LayoutParams(
                RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT
        );
        centerTableParameters.addRule(RelativeLayout.CENTER_IN_PARENT);

        LinearLayout llayout = new LinearLayout(this);
        llayout.setOrientation(LinearLayout.VERTICAL);
        llayout.setGravity(Gravity.CENTER);

        Resources res  = getResources();

        Intent in = getIntent();
        int index = in.getIntExtra("com.example.ITEM_INDEX", -1);

        if(true){


            // create tables:
            boolean tablesEnd = false;
            boolean lineEnd = false;
            int j = 0;
            int i = 0;
            while(i < cn.length){
                TableLayout tableLayout = new TableLayout(this);
                TableRow tableRow1 = new TableRow(this);
                TableRow tableRow2 = new TableRow(this);
                TableRow tableRow3 = new TableRow(this);
                TableLayout.LayoutParams lp = new TableLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);

                tableLayout.setLayoutParams(lp);

                // create lines:
                lineEnd = false;
                while(i < cn.length && lineEnd == false){
                     [...]
                }

                tableLayout.addView(tableRow1);
                tableLayout.addView(tableRow2);
                tableLayout.addView(tableRow3);

                llayout.addView(tableLayout, centerTableParameters);
                Space space = new Space(this);
                space.setLayoutParams(new LinearLayout.LayoutParams(10, 20));
                llayout.addView(space);

            }

            // old:
            SView.addView(llayout);
            setContentView(SView);

            // new:
            //activityLL.addView(llayout);
            //setContentView(R.layout.activity_lesson_view);
        }
    }

如果我运行使用 "old" 代码（请参阅注释 "old" 下方的行），它运行没问题。旧代码不使用任何布局文件。我想在布局中添加更多功能；）一旦我使用 "new" 行（旧行作为注释）启动 activity，应用程序就会崩溃并显示错误消息：

2019-10-26 23:51:42.694 4599-4599/com.example.MyApp E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.MyApp, PID: 4599 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.MyApp/com.example.MyApp.LessonView}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference at com.example.MyApp.LessonView.onCreate(LessonView.java:242) at android.app.Activity.performCreate(Activity.java:7136) at android.app.Activity.performCreate(Activity.java:7127) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

错误似乎发生在 "activityLL.addView(llayout);" (line 242) 但我没有解释为什么...

Answer 1

在使用 LinearLayout activityLL = (LinearLayout) findViewById(R.id.myLL);

检索 LinearLayout 之前，您应该调用 setContentView(R.layout.activity_lesson_view);

Android 动态添加 Scrollview-Content

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